1. Field of the Invention
This invention relates to an optical recording medium which has a recording layer comprising a dye film, and a method for overwriting such medium. More specifically, this invention relates to an optical recording medium which can be recorded and reproduced with a light of a short wave length (600 to 680 nm), and a method for overwriting such medium.
2. Prior Art
A recording method of an optical recording medium which has gone into actual use is heat mode recording method wherein a laser beam is directed to the recording layer as heat source to induce evaporation, decomposition or the like in the recording layer to thereby form concave pits which are detectable by an optical means.
A wavelength in the vicinity of 780 nm is used in such method for the recording and the reproduction, and use of a light with shorter wavelength is required to fulfill the need of the higher density recording.
With the progress in the recording/reproduction using a laser of shorter wavelength, reading out the information that has been recorded with the conventional laser wavelength of approximately 780 nm by a reproduction system using a laser with a short wavelength of about 630 to 680 nm, and vice versa are now required. In other words, there is a need for compatibility between the recording/reproduction at the conventional wavelength and the recording/reproduction at a shorter wavelength.
Such recording/reproduction of the optical medium at a higher density and use of a laser beam with a shorter wavelength also requires change in the type of the organic dye in the recording layer and change in the groove configuration.
Japanese Patent Application Kokai No. (JP-A) 81966/1997 discloses an optical recording medium wherein a U-shaped groove with a depth of 140 to 200 nm and width of up to 0.40 .lambda./NA .mu.m (.lambda. is wavelength of the recording laser beam, and NA is numerical aperture of the laser beam focusing lens) is used for the guide groove. In the investigation of the inventors of the present invention, however, the recording and reproducing properties of this medium has been insufficient.